1. Field of the Invention
The present invention relates to wireless communications systems, and more particularly, to a system and method for blind estimation of multiple carrier frequency offsets and separation of user signals in wireless communications systems.
2. Related Art
In wireless communications systems, carrier frequency offsets (CFOs) represent a severe problem which can make data transmission highly unreliable. CFOs are often caused by two different factors, namely, carrier frequency mismatches between local oscillators (transmit and/or receive) of transceiver equipment, and Doppler shifts caused by moving transceiver equipment (e.g., mobile cellular telephones). Carrier frequency mismatches occur when transmitter and receiver local oscillators experience drifts from their nominal frequencies, resulting in an offset. In multiple antenna systems, each transmitter and receiver typically requires its own radio frequency—intermediate frequency (RF-IF) chain, resulting in each transmitter-receiver pair having its own CFO and associated mismatch parameter. This multiple frequency offset can occur in wireless sensor networks, as well as in multi-user and multi-antenna communications systems where multiple transceivers, positioned spatially apart from each other, are provided and do not share RF-IF chains.
In mobile wireless systems, Doppler shift of the received signal spectrum arises from relative motion between two transceivers (e.g., motion of a cellular telephone with respect to a base station). This shift depends on the carrier frequency, the velocity of the mobile terminal, and the angle of arrival of the received signal. Often, multiple-access wireless systems (e.g., systems with multiple user signals propagated over a shared communications channel, such as in CDMA systems) are used in demanding propagation environments with rich scattering and large angle spread. As a result, each channel branch introduces its own Doppler shift which requires compensation.
Uncompensated CFOs cause undesired channel variations, rotation of the received symbol constellations, and interference in adjacent channels. Compensation of CFOs is particularly important in multi-user and multi-antenna systems, where susceptibility to such problems is high. In such systems, the received signals represent co-channel signals that are mixed because of unknown channel conditions present in the transmission environment.
CFO compensation and signal separation processes are typically performed using training signals. However, such systems are impractical in systems with multiple transceiver pairs because of the need to provide a separate training signal for each transmitter-receiver pair, which is costly and time-consuming and reduces the effective data rates. Additionally, a multi-antenna system is usually required in order to compensate for multiple CFOs and to separate multiple user signals, which results in increased hardware costs. Other techniques for compensating for CFOs include decision feedback via a phase-locked loop (PLL, which uses knowledge of the transmitted constellation to adaptively track both the frequency and phase offset between the equalized signal and the known signal constellation), blind estimation of CFO and recovery of symbols using second-order cyclic statistics of an over-sampled, received signal, and pilot-based CFO estimation. However, such systems are impractical for CFO compensation and user separation in multi-user systems, and particularly, multi-user systems which utilize a single receive antenna.
Accordingly, what would be desirable, but has not yet been provided, is a system and method for blind estimation of multiple carrier frequency offsets and separation of user signals in wireless communications systems, which addresses the foregoing limitations of existing wireless systems.